An optical device includes an input/output optical coupler, a waveguide and a waveguide fragment. The optical coupler is configured to separate a received optical signal into first and second signal components. The waveguide is connected to the optical coupler and configured to propagate the first signal component via a first propagation mode. The waveguide fragment is located adjacent to the first waveguide and is configured to couple light from the first waveguide that propagates therein by a different second propagation mode.